Heat treating apparatus



Sept. 12, 1933. Jjw. HARscH HE'AT TREATING APPARATUS Filed Sept. f7. 1932 2 Sheets-Sheet l INVENTOR. 5% #MJ By X. @gli L ATTORNEY.

Sept. 129 i933. J. W. HARscH 1,925,234

y HEAT '.rREAI'rINcl APPARATUS l Filed sept; 7, 1932 2 sheets-'sheet 2 10 J No CHROM/UM N COPPER INVENTOR.

ATTORNEY.

Patented Sept. 12, 1933 dUNITI-13 STATES- PATENT OFFICE 1,926,234 HEAT TREATING APPARATUS Application September 7, 1932 Serial N0.l 632,078

11 claims. (cl. 26e-5) My invention relates to apparatus used for nitriding metals, particularly ferrous metals, steel, alloy steel, etc., especially machined or finished parts as gears, shafts or the like.

In nitriding, it was found that the efficiency of the nitriding apparatus decreased, more and more rapidly with the continued use, and to such extent as to render the cost of the process prohbitively high. I discovered that the components of the treating apparatus, as walls, shielding structure, Work containers, fan blades, etc., were responsible for a parasitic reaction which robbed the work of the gas supplied for nitriding.

Specifically, it was found that these parts themselves not only nitrided but formed catalytically active surfaces which increased in area the longer the furnace was in use and so Wasted more and more of the nitriding gas, requiring undue length of runs and large quantities of nitriding gas per run.

In accordance with my invention, the structural components or parts of the treating apparatus which is exposed to the treating atmosphere, are composed of an alloy which ishigh in nickel, from 20% to 90%, to resist the nitriding action of nascent nitrogen, and whose other constituent or constituents, as chromium, or copper, are in such proportion as to prevent cmbrittlement of the nickel by nascent hydrogen, the other component of ammonia gas used innitriding as a source of nitrogen. If 'iron is used as one of thev constituents, its permissible maximum proportion is greater in the nickel-chrome alloys than r in the nickel-copper alloys.

In general, a suitable alloy is 20% to 90% nickel, copper or chromium 10% to 40%, and if containing iron, the iron content should not exceed for the nickel-chrome alloys, or 40% for the nickel-copper alloys.

For an understanding of my invention, reference is to be had to the accompanying drawings, in which:

Fig. 1. illustrates one of the various forms of nitriding apparatus utilizing my invention.

Fig. 2 is a chart giving the range of proportions for nickel-chrome,'or nickel-chrome-iron alloys.

Fig. 3 isa chart giving the range of proportions for the nickel-copper-iron alloys.

There is shown in Fig. 1A a furnace or heating structure comprising an outer cylindrical shell 1 having its lower portion closed by a bottom plate 2 suitably secured to the lower edge of shell 1, as by bolts 3.to flange 4. To the bottom plate 2 is secured the furnace supporting structure 5.

`The upper edge of shell 1 is united to a similar shell 6 having an inwardly turned ange 7- whose inner edge is secured and sealed, as by weld 8, to the upper edge of a shell 9, disposed concentrically within, and spaced from shell 1 by insulating material 10 of suitable characteristics. Shell 9, which comprises the inner lining of the furnace, is open at its upper end, and is closed at its lower end by members 11 and 12, members 9, 11 and 12 being secured to each other by the hermetically sealedwelded joints 13. Member 12, comprising the bottom portion of the furnace chamber, is secured and .spaced with respect to plate 2 by members 14 which are also welded to member 9 for purposes of sealing. Upright supports 15, comprising angular members, are mounted upon the bottom of the furnace chamber by means of studs 14a, and support a plurality of heating elements, as electrical resistors 16, byineans of annular supportingbands 16a and insulators 16h. Mounted also upon supports 15 is a cylindrical shell-like member 17, open at its top and bottom, and within which a work container 18 vis concentrically' disposed. The work container 18 comprises a cylindrical shelllike wall having an open grill work or spider 19 secured to the lower edge thereof, and an annular supporting fiange 20 secured to the upper edge thereof adapted to seat upon the upper edge of cylinder 17, and thereby closing the intervening space from circulation of gases. Supporting flange 20 is provided with eyes or handles 21 for facilitating withdrawal of the work container from the furnace.

Suspended beneath the furnace by means of structure 22 and through bolts 22a is a motor M having the axis of its rotor shaft disposed in a vertical plane. Rotor 'shaft 23 extends through the bottom plates 2 and 12, and is substantially sealed with respect to the furnace Walls by means of a packing gland 24-and a sleeve or collar 25 forming a part of said gla'nd and welded to the bottom 4member 12 at 26. Shaft 23 terminates beyond a conical iiow-deflecting member 27, and has secured to 'the end thereof a fan or impeller 28.

Resistors 16 are connected to an external source of electro-motive force bymeans of lead members 26' and 27 suitably mounted and protected within conduit or housing member 28 extending through the wall of the furnace. The inner end of conduit 28 is welded at 28a to the co-actng edge of shell 9, and is provided with an electrical insulating bushing 29. Conduit 28 has mounted at its outer end a member 30 closed by a plug 31 at its lowerend and supporting a sleeve-like member 32 for supporting the. lead conductor 27. The conducting members 26 and 27 are suitably spaced and insulated wrlth respect to their supporting members, as by an electrical insulating bushing 33, asbestos packing 34 and an insulating cement 35 which also serves to insure sealing of the conduits from the exterior against Aso gas leakage. An electrical insulatingbushing '36 and a lead terminalv 37 are disposed at the end of sleeve 32. Although but one outlet supply .conduit has'been described and illustrated, it

will be understood that a proper number of them are supported by and extend through the furnace wall, for supplying current to the resistors, depending upon the power characteristics desired. Cover structure for the furnace comprises a cylindrical shell-like member 38 of somewhat larger diameter than member 6, having the dished-plate-like members 39 and 40 suitably secured thereto, member 39 being welded at its outer periphery to member 38 for purposes 'of hermetically sealing'the junction of these members. Members 39 and 40 are spaced from each other forming an enclosure for insulating material or" suitable characteristics, and have mounted 'therein' a thermo-responsive device 41 comprising a supporting tube 41a`we1ded onits outer surfaceat 41h to member 39 and sealed by suitable cement or equivalentat 4lc. The therrnc-responsive device 4l, as a thermo-couple, and a fluid conduit 42, extend through the cover structure to communicate with the furnace iriterior. Conduit 42, welded at 42h to member 39, is connected to a flexible tube or conduit 43 having interposed therein a vmeasuring device, as a flow meter 44 and a flow controlling device, as a vneedle valve 45 for controlling the flow from a supply conduit 46.

The lower portion of shell 38 comprises a sleeve-like structure surrounding the outer wall of the furnace and `co-acting with a liquid seal, as an oil seal 47 for hermetically sealing the furnace chamber with respect to the atmosphere.'

lresilient material, such as asbestos rope 49, is

disposed along the upper edge of the furnace wall to provide a seat for the above described cover structure. The cover structure, including the thermo-responsive device 41' and the conduit 42, may be lifted as a unit from the furnace by hoisting means connected to the lifting hook or eye 50.

The furnace is provided with an outlet or ex` haust conduit 51, welded at51atc the furnace lining, having seated at its outer end a gasketed closure member or plug 52 adapted to be unseated to open the outer end ofthe conduit by a member 53, pivoted at 5 4, and connected to 'the closure member through a pivoted connection 55. The closure member 52'may be maintainedV in snug and sealing engagement with the end of conduit 51 by means of the screw clamp 56 pivotally mounted at 57.

The material of parts of the interior furnace' structure, such as shell 17, container 18, spider 19, supports 15, supporting flange 20, top member 39, fan 28, flow-deecting member 27, and lining 9, 1l and 12, etc., exposed to contact with the particular uid medium utilized the treating process, comprises an alloy substantially unaiected by the temperature and/or the nitriding atmosphere. l

Heretofore chrome-iron, about 13% chromium, remainder iron, and chromium-iron-nickel alloys of low nickel content, for example chromium .18%, iron 74%, Aand. nickel 8% were vused for the walls, for example, lining 9, shell 77 etc. These elements and others of like sheet material after continued use of the furnace became nitrided to an extent effecting buckling and warping, the continued adsorption of nitrogen by the walls increasing the dimensions'of the nitrided surfaces thereof. But even long before buckling or warping requiring replacement of these parts, the quantity of gas absorbed by these elements and other furnace` parts'of like material as the work container 18, fan 28, supports 15 etc., during a run, or treatment of a batch of work, was so great as to make the cost excessively high. With continued use, more and more treating gas and more and more heat energy per run was required and as the length of the runs'became longer and longer, production was lcurtailed because of reduced ,effective capacity of the treating apparatus.

I found that the nitrided surfaces of Vthe parts were catalytica'lly active inducing dissociationof the ammonia at these parts instead of at the work, the area of the catalytic surface increasing with continued use of the furnace and so more and more robbing the work of active treating gas. More and more of the gas'was uselessly expended in nitriting the furnace parts and less and less of the gas wasavailable for useful nitriding of the work.

Nickel resists the nitriding action of nascent nitrogen but succumbs to hydrogen which is released in its nascent state upon dissociation of ammonia.

In accordance with my invention, a compromise is resorted to by utilizing, for the structural parts of the furnace or treating apparatus exposed to ammonia, and its dissociated-elements hydrogen and nitrogen, an alloy which is high in nickel. andl whose other component or components preclude embrittlement by hydrogen.

By recourse to such an alloy, the structural parts exposed to the treating medium, are neither embrittled by the hydrogen, nor substantially if at all nitrided, with the result that the operating eiliciency of the apparatus remains high for indefinitely long periods and that there is no need for replacement of these parts, requiring in many i instances rebuilding of the furnace. s

The absence of nitriding of these parts, and the avoidance of catalytically active surfaces thereon ensures that all or substantially all of the dissociation of the ammonia shall occur at and adjacent the work under treatment. As a result,

the desired nitriding is eected in shorter time Las 'and should preferably be less and the nickei 56 should not be less than In general, the alloy is better for the proportions farther to the left of the shaded area of Fig.` 2, although all proportions given are practical and much superior to the aforesaid materials previously used. From the figure, for any given percentage of nickel, the proper percentage or percentages of chromium, or chromium and iron may be determined.. For example, if the alloy is to be 70% nickel, the chromium-Will be %-10% and the iron 0%-20%. It is also apparent from this chart, that the chrome-iron, and chromium-ironnickel alloys of the proportions previously` used and found unsatisfactory are substantially outside the shaded area representative of the suitable range of proportions. Point N corresponds to chrome-iron having 13% chromium and remainder iron; point G corresponds to a chromium-iron-nickel alloy of chromium 18%, iron 74%, and nickel 8%. Both of these alloys were found to be wholly unsuited and responsible for the difficulties encountered in attempting to nitride with furnaces whose internal structure was made of these alloys.

Referring to Fig. 3, for illustration of the suitable range of proportions for nickel-copper, the nickel content is again high, between 60% and 90%, and the copper iron lies between 10% and For the nickel-copper alloys, the permissible maximum of iron is less than for the nickelchrome-iron alloys and should not exceed 40% and the minimum of nickel should not be less than 35%. From the figure, for any percentage of nickel, the proper percentage of copper, or percentages of copper and iron can be readily cletermined. In general, the farther to the left within the shaded area that the point identifying the alloy lies, the better it is suited. The point M corresponds to Monel, a nickel-copper-iron alloy of the order of nickel, 33% copper and 6% iron. Monel containing manganese, which is sometimes added, should be avoided, because it is conducive to nitriding.

As shown by the charts of Figs. 2 and-3, the

proportions of nickel and other component or components may be varied substantially Within the limits of the shaded areas of Figs. 2 and 3, for which the proportion of nickel 'to the other component or components is such that there is no substantial embrittlement by hydrogen, and keeping in mind that the nitridible component or components, if any, shall not be present in such proportion to permit substantial if. any nitriding. 1

The heating resistors are preferably a nickelchromium alloy, the proportions of nickel and chromium, lying within the range of the chart of Fig. 2, i. e., 60% to 90% nickel and 40% to 10% chromium.

The liquid seal at the exterior of the furnace is substantially at room temperature and is not appreciably affected by the high temperatures existing within the furnace, 'thereby preventing rapid deterioration or evaporation of the sealing fluid.

The operation of the system is as follows:

Assuming the furnace to be in condition for recharging, the cover structure is removed by suitable lifting or hoisting mechanism secured to member 50, after which work container 18 is lifted from the furnace by means of eyes 21. A work container filled with material to be treated is subsequently lowered into the furnace to the position shown in Fig. 1, after which the cover structure is lowered into its sealing position. In

the outer annular heating chamber containing resistors 16. In this manner, the treating gas is i caused to circulate throughout all parts of the furnace interior to effectively flush out all gases such as air or other oxygen-containing gases, and to effect discharge of the same through exhause conduit 51 and the liquid seal 60. During the above described fiushingprocess, resistors 16 are deenergized and generate no heat, so that the new batch of material to be treated remains comparatively cold and is not acted upon to any appreciable extent by the flushing treating gas. Since the treating gas is generally introduced into the interior of thefurnace at comparatively low pressure, the pressure therein during normal operation will be somewhat above atmospheric, so that there is a continuous flow of gas through the exhaust conduit and seal, thereby insuring a continuous supply of. fresh treating gas. 1J5

After the flushing process has been completed, resistors 16 are energized from a source of power (not shown), and the furnace interior accordingly increases in temperature. Assuming now that fan 28 rotates in such direction that the furnace gas is caused to flow upwardly through the open grill or spider 19 of the work container 18, through the batch of Worktherein, the gas will return to fan 28 by way of the annular heatlng chamber, and flow around and contact with the highly heated resistors during the downward return path to fan 28, Where it is guided by the conical deecting member 27 towards the fan blades which again lmpel the gas through the same cycle. By reversing the `motor M, and consequently the direction and rotation of fan 28, the gas or gases flowing directly from the heating chamber first come into contact with the work at the top of the container 18, instead of that at the v bottom, asin the previous instance.

The above described method of periodically reversing the direction of circulation of gas within a furnace effects uniformity of heating of the work therein, and is fully described and claimed `in my Patent 1,578,027, March 23, 1926.

gases. itself is a determining factor in governing the rate and/or extent of reaction of the treating gas with the work. By utilizing the treating gas itself as the principal heat vehicle between the vfurnace through the exhaust seal such exhausted In general, the temperature of the work e source and the work, the cylindrical shield 1.7 to- M5,

gether with container 18, both of which are unperforated, effectively' preventing appreciable transmission of radiant heat to the work, it is possible to effect gradual and uniform heating of the work while at the same time subjecting it to the even and uniform ow of the treating gas or gases, whereby the gas and` the metal or material to be treated are concurrently in heattransfer and chemically-reacting relations.

Although no definite rate of circulation of thecombinedheat vehicle and treating gas `is contemplated, the fan or impeller 28 should rotate at such speed that the circulating medium effectively removes or wipes olf stagnant films on the surfaces of the work under treatment, in order that the rate of treatment may be materially increased, instead of being reduced by the f not create a vacuum within the furnace interior and so cause the liquid within seals 47 and 60 to be drawn into the furnace. To this end, sealing plug 52 is released by unloosening clamping nut 56, after which lever 53 is rotated in a counterclockwise direction to move plug 52 out of sealing engagement with the end of conduit 51. Accordingly, the interior of the furnace is now directly in communication with atmosphere and hoisting of the cover structure cannot therefore create a vacuum within the furnace to break and disrupt the aforesaid liquid seals. It is essential that the sealing plug 52 be open only while the cover structure is being lifted from the furnace to permit removal of the work, since replacement of the cover would only tend to force excess air through the exhaust seal.

An example of the use of a chemically active gas for effecting concurrent heat and chemical treatment-of a metal lies in the use of ammonia, part of which is dissociated into` nitrogen and hydrogen for the nitriding of steel. As is well known in the art, a nitrided steell or suitable alloy thereof has valuable wear-resisting characteristics, the

-nitrided material having a very hard outer surface or shell highly resistant to wear or abrasion. An example of a practical use to which a nitrided steel may be put, lies in its application to shafts or equivalent members incorporated in high speed machinery, lsuch asin automotive engines.

Before the nitriding treatment is actually started, ammonia gas or vapor is caused to circulate,

in the manner previously described, by the fan for an appreciable length of time, generally about an hour, through the treating chamber and other parts of the furnace to the exterior through the exhaust seal, in order that air or any other undesirable gas, as an oxygen-containing gas for example, shall be completely forced out of the furnace. During this gas :Bushing period, the batch of metal to be treated is not acted upon by the ammonia, since the electrical` resistors are deenergizedf After the flushingperiod has been completed, the circuit through the resistors is closed, and the furnace interior accordingly Vincreases in temperature. As the fresh ammonia gas is introduced under pressure into the upper portion of the furnace, it is caused to flow downwardly either through the treating chamber on theheating chamber, depending on the direction of rotation of the fan, and to circulate between the source of heat and metal. As the ammonia gas, which comprises the heat vehicle, brings the metal up to the desired temperature, dissociation i of the ammonia occurs during its contact with the heated work, the nascent nitrogen reacting therewith to form the desired case.-

An individual charge of ammonia would become exhausted within a comparatively short period of treatment, a continuous supply, controllable by the furnace operator or suitable control mechanism, is admitted to the furnace 'through inlet4 42 as weakened or exhausted gas is discharged from the furnace through the yielding exhaust seal to atmosphere,'or to gas recovery apparatus. f

Y The use of alloys of the character above described is all the more important when the treating gas is circulated for the -circulation which is essential to-rapid and uniform nitriding of the heat-transporting medium, and/or in which the heat is developed by fuel-firing etc., the temperatures developed being usually in excess of 900 F.

This application is a continuation in part of my copending application Serial No. 370,493, iiled June 13, 1929 and subsequently abandoned in favor of this case.

What I claim:

1. A nitriding furnace having structure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, -said structure being of an alloy comprising nickel not more than and not less than 20%, to prevent nitriding, with which is alloyed chromium 10% to 40% to preclude embrittlement. Y'

2. A nitriding furnace having structure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, said structure being of an alloy consisting of nickel 90% to 20%, chromium 10% to 40%, and iron not to exceed '70%.

3. A nitriding furnace having structure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, said structure being of an alloy comprising nickel not more than 90% and not less than 35% to prevent nitriding, and copper 10% to 40% to preclude embrittlement.

4. A nitriding furnace having structure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, said structure being of an alloy consisting of nickel 35% to. 90%, copper 10% to 40%, and iron notto exceed 40%.

5. A nitriding furnace having structure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, said .structure being of an alloy of nickel about 60%, copper about 33%, and iron about .6%. l

6. A nitriding furnace having structure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, said structure being of an alloy of nickel 60% to 90%, to prevent nitriding, and chromium 40% to10% to prevent embrittlement.

7. A nitriding furnacehaving structure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, said structure being of an alloy comprising nickel 90% to 35%, to

furnaces, etc., in which the treating gas is not f prevent nitriding, and copper or chromium 10% to 40%, to prevent embrittlement.

8. A nitriding furnace havingstructure subject at furnace temperature to a furnace atmosphere containing nascent nitrogen, said structure being of an alloy comprising nickel 90% to 35% to prevent nitriding, copper or chromium to 40%, and iron not to exceed 40%. u

9. A nitriding furnace comprising a heating zone and a work-nitriding zone, means for circu-y lating ammonia through said zones, and furnace structure exposed at furnace temperature to said circulating gas and consisting of nickel not more than 90% and not less than 20% and chromium 10% to 40%, to preclude catalytic dissociation of said circulating gas external to said worknitriding zone. i

10. A nitridirig furnace comprising a heating zone and a work-nitriding zone, means for circulating ammonia through Asaid zones, and furl 'and iron not to exceed 40%.

JOHN W. HARSCH. 

